New Path class that makes building/manipulating URLs easier

I've added a new Path class that lets you work with the URL in a more resource-oriented manner by letting you build it with either methods or an options hash. Check out the updated README for some examples.

I've maintained backwards compatibility with the `sign_path` method so nothing will break in the transition. I've also added some new tests for the additions. Ruby 1.8 compatibility should be maintained if all goes well.